Alkyl-2-methoxypyrazines (MPs) represent an important and potent class of grape- and insect-derived odor-active compounds associated with wine quality. Specifically, 3-isobutyl-2-methoxypyrazine (IBMP), 3-isopropyl-2-methoxypyrazine (IPMP) and 3-secbutyl-2-methoxypyrazine (SBMP) are 3 grape-derived volatile compounds that elicit green and vegetative aroma and flavour descriptions in wine. Although these MPs can positively influence wine quality in some varieties (1), at higher concentrations they are dominant and unpleasant (2), can mask “fruity/floral” aromas (3), and are associated with cooler climates and under-ripe, low quality fruit (4-6).
Recently, a second source of elevated MPs in wine has been recognized that is responsible for ladybug taint (LBT; (7)). LBT is a wine defect resulting from the undesired incorporation of ladybeetles (Coleoptera: Coccinellidae), particularly Harmonia axyridis, into the fermentation process and is responsible for millions of dollars of lost revenue from downgraded or discarded wine in Southern Ontario and parts of the USA (8). The deleterious effects of the Multicolored Asian Lady Beetle (MALB) on grape juice and wine quality were first noticed in Ontario in 2001, where up to 50% of the wine produced from that vintage was considered tainted or not fit for sale. The prevalence of Harmonia axyridis in other wine regions, including Italy, France, Spain, South Africa and Argentina (9), suggests that LBT could be or become a more widespread problem for the wine industry.
Regardless of source, MPs are present in wines in trace amounts and due to their extremely low detection thresholds—in the high pg/L to low ng/L range (4, 10, 11)—have the potential to significantly lower wine quality resulting in lost revenues to grape and wine industries. Efforts to reduce MP levels have included both viticultural (12, 13) and oenological (14) interventions. However, attempts to decrease MP concentrations in wine using conventional treatments, such as fining, have had limited success (15).